The present invention relates generally to the continuous casting of free-machining steels and more particularly to a method for preventing mold explosions during the vertical continuous casting of molten steel containing at least one of lead and bismuth.
In a vertical continuous casting operation, molten steel is flowed from a ladle into an intermediate container called a tundish having a plurality of spigots in its bottom through which molten steel is flowed into a vertically disposed casting mold having an open upstream end and sidewalls extending from the upstream end to a downstream end initially closed by a so-called "dummy bar" at the bottom of the mold. The sidewalls of the mold are cooled, e.g. with a water jacket, and the molten steel initially contacting the dummy bar and the side-walls solidifies to form a solid shell enclosing unsolidified molten steel. After this initial solidification forming a solid shell, the dummy bar is withdrawn from the mold's downstream end, and the solidified shell containing unsolidified molten steel advances through the mold in a downstream direction, e.g. under the urging of gravity. The portion of the mold vacated by the shell's downstream movement is replenished with additional molten steel.
To facilitate the movement of the partially solidified steel through the mold and to prevent the solidified steel shell from sticking to the inside surface of the mold, a lubricant is employed. The lubricant is typically an organic compound such as a petroleum-base oil, and the lubricant is typically applied to the interior surface of the mold before the casting operation begins. Lubricants are also injected between the molten steel and the interior surface of the mold walls during the casting operation. Typical examples of lubricants employed during the continuous casting of molten steel are set forth in Borg, et al U.S. Pat. No. 4,120,344.
There are problems which occur during the continuous casting of steel containing machinability increasing ingredients, such as bismuth, lead and tellurium, which do not ordinarily occur during the continuous casting of molten steel not containing these ingredients. In the continuous casting of molten steel containing any of these ingredients, explosions occur within the mold when the mold is lubricated with conventional lubricants.
Some attempts have been made in the past to solve these problems. Thus Borg et al, U.S. Pat. No. 4,120,344 teaches the prevention of explosions in tellurium-containing steel by employing, as the mold lubricant, a white mineral oil. Although this expedient prevents mold explosions during the continuous casting of tellurium-containing molten steel, it does not prevent mold explosions during the continuous casting of molten steel containing bismuth.
Engeler, U.S. Pat. No. 4,165,780 discloses a method for preventing mold explosions in the continuous casting of molten steel containing lead. Engeler teaches that mold explosions can be avoided by eliminating the use of oil as a mold lubricant and by employing, as the mold lubricant, a mixture of a liquid inert gas (e.g. nitrogen) and a flux powder (e.g. carbon particles) applied atop the molten steel in the mold, while at the same time oscillating the mold back and forth along the path of movement of the steel through the mold. The mixture of inert gas and flux powder forms a layer atop the molten steel, and the thickness of the layer is controlled so that it is less than the length of the stroke of the oscillating mold. The Engeler method is relatively complicated and requires continuous monitoring.